A coarse, colloquial imperative meaning 'don't mess around' or (depending on context) 'screw you'; expresses strong anger or scorn.
ふざけんなよ！そんな簡単に決めるな。
Don't mess around! Don't make that decision so easily.
彼の冗談に腹が立って、思わず『ふざけんな』と言ってしまった。
I got angry at his joke and blurted out, 'Don't mess around.'
警察に向かって『ふざけんな、何様のつもりだ』と叫ぶのはよくない行為だ。
Shouting 'Screw you, who do you think you are?' at the police is not appropriate behavior.
日常会話で使うと人間関係が壊れることがあるので注意が必要だ。
Be careful: using it in everyday conversation can damage relationships.